This invention relates to thin-film EL panels and more particularly to a mask for use in the fabrication of back metal electrodes thereon.
Thin-film electroluminescent (EL) panels are very useful for displaying information inasmuch as the thin-film structure including the cross electrodes therefor can be deposited on a glass substrate thereby providing a matrix display panel capable of being selectively energized to activate individual pixels thereon.
Heretofore, thin-film small EL panels having a resolution of, for example, 56 electrodes per linear inch have been made by placing a thin flexible steel mask over the back surface of the substrate which already has the front transparent electrodes and thin-film structure deposited thereon. The thin flexible steel mask is held flush against the thin-film structure provided on the substrate by a permanent magnet enabling an accurate replica of the mask pattern to be obtained thereon by the vacuum deposition of a metal through the openings thereof. Such an approach is feasible for small, one square inch, EL panels because the filaments on the mask are relatively short.
However, when making larger EL panels, such as 2" by 3" or larger in size, and having a resolution on the order of 56 electrodes or more per linear inch, the closely spaced parallel thin filaments, formed on the mask to define the back metal electrodes, are relatively long. Accordingly, inasmuch as the thin filaments are being supported only at their ends by the frame of the mask, they become distorted sidewise, i.e., bowed along their length, by the magnetic field of the permanent magnet used to hold the mask against the substrate. Consequently, the openings on the mask do not properly define the parallel elongated metal electrodes on the EL panel. Moreover, when the end supported filaments on the mask are relatively long, the intense heat of the tungsten heater coil used to vaporize the metal in the vacuum chamber is radiated to non-uniformly heat the filaments thereby causing distortion or expansion thereof, i.e., causing them to bend outwardly or buckle away from the plane of the substrate in spite of the pull of the magnet. This causes the metal vapor to settle on the substrate behind the filaments so as to not define the metal electrodes but rather continuous areas of metal resulting in some of the electrodes being shorted.